A semiconductor device can achieve a high breakdown voltage and a low on-state resistance effectively by using a material that has a high critical electric field. For example, a high electron mobility transistor (HEMT) using a nitride semiconductor exhibits a high breakdown voltage and a low on-resistance by utilizing a two-dimensional electron gas (2DEG) generated at a heterojunction interface.
An electron-hole pair is formed by a phenomenon called impact ionization in which an accelerated electron excites an electron in a valence band, when a field effect transistor using a nitride semiconductor is operated under high voltage. The hole generated is stored in an electron channel layer and causes a kink effect in an electrical characteristic of the transistor. The stored hole further induces avalanche breakdown, thereby leading to a drop in the breakdown voltage. It is therefore desired that a semiconductor device has a structure capable of absorbing the stored hole more efficiently.